The present invention relates to the field of control of AC motors and in particular to control of AC motors used in vehicles.
Japanese Kokai Patent Application No. 2004-72954 discloses a conventional motor control device, in which the DC power of a battery is converted into AC power and supplied to a motor. Switching is performed between square-wave voltage driving that applies a square-wave voltage to the motor for driving control and pulse width modulation (PWM) wave voltage driving that applies a PWM wave voltage to the motor for driving control corresponding to the rotation speed of the motor and a torque instruction value.
Since square-wave voltage driving has a higher duty ratio than PWM voltage driving, the voltage needed for generating the same torque is lower than that of PWM voltage driving. Consequently, in the aforementioned conventional motor control device, when PWM voltage driving is switched to square-wave voltage driving, the current sharply increases instantaneously. As a result, the torque increases significantly, causing a shock.